[vtkusers] Is there a concept of "empty" data?

[Phil Goddard]

Is there the concept of "empty" data within VTK?

Specifically, I have points that are defined on a rectangular grid, and
would like to use vtkImageData for its efficient use of memory.
However my image does not have values at all points of the grid.

I'm used to using an application (MATLAB) where if an element of my data is
set to NaN (which is an IEEE standard for Not a Number -- i.e. a placeholder
in a numeric array that is not a number) then that point does not get
visualized and no adjacent points will get drawn as connecting to that
point.
Is there a similar concept with VTK?

I think I have to resort to using vtkPolyData or vktUnstructuredData, but
want to check that I'm not missing something with regards to vtkImageData
first.

Thanks
Phil.